Royalty Free Sounds are sounds that are free from royalties. They can be used commercially in things like movies, games, and anything else you might need a cool sound for. You may not however redistribute them for a profit. While sending one to a friend is ok, selling one to a customer is not.

Public Domain Sounds

Public Domain Sounds are sounds that have been added to the public domain. They are owned by the public, and are 100% free. Items get added to the PD in a variety of different ways. 1.) Items older than the public domain age setting are automatically added. 2.)Thoughtful members of the audio community donate their own works for the good of others. 3.) Speeches by presidents and other political audio is usually considered public domain works.